1. Field of the Invention
Embodiments of the present invention generally relate to a method of using an atomic force microscope (AFM), and more particularly to a method of determining a feedback setpoint value for use in operating the AFM.
2. Description of the Related Art
The surface roughness and morphology of gate dielectrics and other semiconductor thin films are critical properties that need to be controlled to ensure integrated circuit device integrity and reliability. These properties can be measured by an atomic force microscope (AFM) operated in dynamic mode. In dynamic mode operation the AFM tip is oscillated near the cantilever resonance frequency with the oscillating amplitude and phase monitored by a detection mechanism and AFM electronics.
As the AFM tip gradually approaches the sample surface, it will first enter a van der Waal force field (the so called non-contact attractive force interaction regime) or other attractive force fields, such as magnetic, capacitive, electric, friction, lateral, and capillary (mediated by a condensing vapor such as water) fields. Because the van der Waal force field is the most prevalent one, the discussion of the invention will be primarily focused on this force field, even though the invention can also be applied to other attractive force fields. Due to the attractive force field, the oscillating amplitude will monotonically decrease, and the phase signal will also monotonically decrease (or increase, depending on the implementation with different phase detection electronics) with decreasing tip-to-sample distance. Further decrease of the tip-to-sample distance will cause intermittent hard contact between the tip and the sample surface along with an abrupt and sharp reversal of the phase signal.
In practical roughness measurements and morphology imaging by an AFM tool, the amplitude decrease or phase change (increase or decrease) due to tip-to-sample force field interaction will be maintained at a constant value by a feedback control mechanism, and a mechanical apparatus will scan the tip laterally and simultaneously move the tip up and down to follow the contour of the sample topography. During AFM operation, the feedback electronics will be provided with an amplitude reference signal or a phase reference signal termed the feedback setpoint, and the feedback mechanism will try to maintain the actual tip oscillating amplitude or phase the same as the feedback setpoint. It is desirable to operate this constant feedback setpoint at a value such that a pure non-contact attractive interaction between the tip and the sample is ensured, thus preventing the AFM tip from even momentarily making hard contact with the sample surface. Such hard contact usually causes AFM tip damage and the loss of accuracy and precision. Therefore, a critical feedback setpoint where the onset of hard contact occurs should be determined precisely, and the AFM will typically be operated at an amplitude setpoint that is above this critical setpoint—or a phase setpoint that occurs before this critical setpoint whether above or below—to ensure non-contact attractive interaction between the tip and the sample.
The traditional method for determining this critical feedback setpoint dividing the attractive force interaction regime and the repulsive force interaction regime is to approach and retract the AFM tip towards and away from the sample surface repeatedly with the feedback mechanism being turned off while monitoring the amplitude and phase signals of the AFM cantilever vibration. When the amplitude feedback scheme is to be employed during subsequent AFM operation, the critical feedback setpoint is determined by the small deviation in the monotonic relationship between the amplitude decrease and tip-to-sample distance decrease. However, in this measurement, the tip is repeatedly rammed into the sample surface with no control at all and the tip will certainly be damaged during the measurement. Due to tip damage, the critical setpoint measured may be grossly inaccurate.
Accordingly, what is needed is a method of determining the critical feedback setpoint value without damaging the probe tip and employing an appropriate feedback setpoint value above the critical value during AFM scanning with an amplitude feedback scheme to remain in non-contact mode.